Heat Transfer

Thermal radiation is the transfer of heat through electromagnetic waves.

Conduuction and Convection transfer heat through particles.

Conduction is the main heat transfer in solids.

Convection is the main heat transfer in liquids and gases.

Emission of thermal radiation can happen is solids, liquids and gases. Any object can both absorb and emit heat radiation, whether or not conduction or convection are also taking place.

1 of 19

Heat Transfer, continued

The bigger the temperature differerence the faster the energy is transfered.

Heat Radiation

Heat radiation can also be called infrared radiation, and it consists purely of electromagnetic waves.

All objects are continually emitting and absorbing thermal radiation.

An object that is hotter than its surroundings emits more heat than it absorbs.

An object cooler than its surroundings emits less heat than it absorbs.

You can feel objects such as car bonnets and fires giving out thermal radiation.

The amount of heat radiated depends on the surface area.

Heat is radiated by the surface of an object.

The bigger the surface area, the more waves can be emitted from the surface, so the transfer of heat is increased.

2 of 19

Heat transfer, continued. Heat radiation

This is why car and motorbike engines have fins, to increase the surface area, therefore increasing the speed they radiate heat.

It is the same with heating things up, the larger the surface area, the quicker it will heat up.

Heat Radiation

Radiation depends a lot on surface colour and texture

Dark matt surfaces absorb heat better than bright glossy surfaces, such as gloss white and silver. They also emit much more heat radiation.

Silver surfaces reflect nearly all of the heat radiation falling on them.

3 of 19

Heat radiation

Making sure heat gets in: Solar hot water panels.

Solar hot water panels contain water pipes under a black surface, or black pipes under glass.

Heat radiation from the sun is absorbed by the black surface to heat the water in the pipes.

Making sure heat gets out: Stoves

Stoves are usually painted matt black so it will radiate as much heat as possible.

Making sure heat doesn't get in: Coolbags

To minimise heat radiation the inside of the coolbag is usually silver or white.

The insulation prevents conduction from happening.

Thermos flasks also have silver inner surfaces to keep heat in or out, depending on the liquid being stored.

4 of 19

Biomass

Biomass:

Biomass is a term for organic 'stuff' which can be burnt to produce electricity.

The material is burnt in power stations to drive turbines and produce electricity.

Sometimes the material is fermented and used to produce biogas.

There is a neutral effect on CO2 levels as the plants had absorbed the CO2 and then as it was burnt it releases the CO2.

Set up costs are usually fairly low as the material can be burnt in converted coal power stations.

Fuel costs are low as it is usually animal waste.

Unfortunatley it only solves some of the problems of quickly filling up landfill sites as it will only accept plant and animal material.

5 of 19

Heat conduction

Conduction of heat occurs mainly in solids.

CONDUCTION OF HEAT is the process where VIBRATING PARTICLES pass on their EXTRA KINETIC ENERGY to NEIGHBOURING PARTICLES.

This process continues throughout the solid and gradually some of the extra kinetic energy is passed all the way through the solid, causing a rise in temperature on the other side of the solid, therefore increasing the heat radiation from its surface.

Metal is a good conductor (This is why it is not used in pan handles very often)

Wood is a poor conductor

Plastic is a poor conductor (mainly used in pan handles)

6 of 19

Heat conduction and heat radiation continued

Metal is a good conductor because of its free electrons.

Metals conduct so well because the electrons are free to move inside of the metal.

At the hotter end, the free electrons have more energy and therefore collide with other free electrons more frequently, resulting in passing on the heat more quickly.

Copper is a great conductor - this is why pan bottoms are made from it.

Making sure heat doesn't get out: Survival blankets

If someone is injured in an awkward place where it potentially may get cold, it is essential to keep them as warm as possible.

A silver blanket is used to prevent their body heat radiating away, potentially saving their lives.

People are also given these silver blankets after marathons to prevent them cooling down too quickly and getting hypothermia.

7 of 19

Useful heat transfers

There is usually more than one type of heat transfer.

Heat conduction through the bottom of a pan is useful as it is heating up the food. Heat radiation around the sides of the pan is not helpful as you are not trying to heat up the air around the pan.

For transfers that are not helpful you want to limit the amount it happens and the more useful it is the more you want it to happen.

For conduction through a pan bottom, you would want to increase the heat transfer, therefore you would use a more conductive material.

For conduction through the pan sides, lid and handle you would use a less conductive material to reduce the amount of heat loss.

To prevent heat being radiated from the pan you would make sure it is made of a shiny silver or white material to reflect the heat back into the pan.

8 of 19

The Thermos flask example

The thermos flask is good at keeping its contents hot or cool because:

The glass bottle is double walled with a vacuum between the two walls. This tops all conduction and convection through the sides.

The walls either side of the vacuum are silvered to keeps heat loss by radiation to a minimum.

The bottle is supported by using insulating foam. This minimises heat conduction to or from the outer glass bottle.

The stopper is made of plastic and filled with cork or foam to reduce any heat conduction through it.

THE VACUUM STOPS HEAT GETTING OUT WILL NOT GET YOU ANY MARKS.

9 of 19

Energy Transfer

The 9 types of energy:

Electrical energy

Light energy

Sound energy

Kinetic energy

Nuclear energy

Thermal energy

Gravitational potential energy

Elastic potential energy

Chemical energy

Potential and chemical are forms of stored energy.

ENERGY CAN NEVER BE CREATED NOR DESTROYED - IT IS ONLY CONVERTED FROM ONE FORM TO ANOTHER.

Energy is only useful when it can be converted from one form to another.

10 of 19

Efficiency of machines

Most energy transfers involve some loss of energy, often as heat.

Useful devices are only useful if they convert energy from one form to another.

When doing so, useful input energy is always lost or wasted, often as heat.

The less energy is wasted, the more efficient the device is said to be.

Calculating the efficiency

Efficiency= Useful energy output

Total energy output

The efficiency can be written as a fraction decimal or percentage.

No device is 100% efficient and the wasted energy is always dissipated as heat.

Nuclear power is very clean, but the nuclear waste is dangerous and difficult to dispose of.

Nuclear power always carries the risk of a major catastrophe

Most power stations use steam to drive a turbine:

The fuel is burnt, which begins to heat up water in a boiler. This water turns to steam and drives a turbine. The turbine then drives a generator, therefore producing electrical energy which is supplied to homes and industry via the national grid.

15 of 19

Nuclear power and renewable energy resources

Nuclear power stations also use steam to drive a turbine:

A nuclear power station is basically the same as the fossil fuel power station, but it uses nuclear fission to heat up the water. Nuclear fission is the splitting of an unstable atom, usually uranium.

Renewable energy resources:

Wind

Wave

Tide a)These will never run out

Hydroelectric b)Most of them do damage the environment

Solar but not as badly as non renewables

Geothermal c)They don't always provide much energy and

Food some are unreliable as they may rely on the

Biomass weather.

16 of 19

wind power

Wind power:

The wind turbines need to be in exposed places such as moors and around coasts.

Each turbine has its own generator so the electricity is generated directly from the wind turning the blades, which turns the generator.

They do not omit pollution

They do spoil views. Around 5000 are needed to replace one coal power station.

They can be very noisy.

There is a problem about when the wind stops there is no power, and it is impossible to increase the supply when there is extra demand.

The manufacturing cost is quite high, but there are no fuel costs, and minimal running costs.

Solar panels are very effective in sunny countries, but are not useable at night as there is no sun

Initial costs are high, but the energy after that is free and running costs are very low.

Solar hot water panels are NOT the same as solar cells. They heat water for household use, no electricity is involved.

18 of 19

Hydroelectricity and pumped storage systems

Hydroelectricity:

Hydroelectricity usually involves the flooding of a valley by building a dam.

Rainwater is caught and allowed out through turbines. There is no pollution.

There is a large impact on the environment, as the flooding of the valley increases the amount of rotting vegetation therefore releasing methane and CO2, and there is a potential loss of habitat for some species.

A great advantage is that there is a great response to increased demand of electricity, and there is no problem with reliability, except in times of drought.

Initial costs are high, but there is no fuel and minimal running costs.

Pumped storage:

Most large power stations have huge boilers that have to be kept running at night even though demand is very low. This leaves a surplus of electricity.

In pumped storage spare "night time electricity" is used to pump water to a higher reservoir.